1. Field of the Invention
The present invention relates to an optical attenuator that uses an element of a micro-electro-mechanical system (MEMS) device, and more particularly to an MEMS variable optical attenuator having an improved optical shutter for regulating optical power of an optical signal by partially intercepting incident light beams.
2. Description of the Related Art
An optical attenuator for use in optical telecommunication networks is an optical component for delivering beams of light passing out an exit end of an optical waveguide to an incident end of an optical waveguide by causing insertion loss to the light beams.
Generally, optical power levels are regulated over wide ranges based on a configuration of an optical telecommunication system. For example, the optical power levels are determined by an optical transmission loss typically varied based on a length of an optical transmission line, the number of connection points of optical fibers, and the number and performance of optical components such as optical couplers coupled to the optical transmission line. An optical attenuator is needed in optical telecommunication networks to reduce an optical power when an optical signal with an excessive power level greater than an allowed power level is received to an optical signal receiver. The optical attenuator further may be used in evaluating, adjusting and correcting telecommunication equipments and optical measurement equipments.
Such optical attenuators are classified into two types, a fixed optical attenuator for reducing an optical power by a fixed amount of attenuation and a variable optical attenuator (VOA) capable of attenuating an optical power by a varied amount of attenuation based on user's requirements. The optical attenuator is required to be produced at low cost with high reliability and small size.
To satisfy such requirements, an optical attenuator that uses an element of an MEMS device has been suggested. Such MEMS optical attenuator is realized by forming a microstructure acting as an actuator on a substrate such as silicon using thin film processing technology. Generally, an MEMS actuator is driven to move by a driving force caused by thermal expansion or an electrostatic force. As the MEMS actuator moves, an optical shutter coupled to the MEMS actuator is displaced so as to be inserted into a gap between two optical waveguides, thereby partially intercepting light beams traveling from a transmitting end (or the exit end) of the optical waveguide such as an optical fiber to a receiving end (or the incident end) of the optical waveguide.
FIG. 1 illustrates a perspective view of a conventional variable optical attenuator using an actuator driven by an electrostatic force. A variable, optical attenuator shown in FIG. 1 includes a substrate 11 with a pair of optical fibers having a transmitting end 20 and a receiving end 30, respectively, an electrostatic actuator comprised of driving electrodes 12a, 12b, a ground electrode 14, a, spring 15 and a movable mass 16, and an optical shutter 17 connected to the movable mass 16 of the electrostatic actuator.
The driving electrodes 12a, 12b and the ground electrode 14 are formed over the substrate 11 and supported by an oxide layer called an “anchor”. The movable mass 16 is connected to the ground electrode 14 via the spring 15 and has a comb shape. The driving electrodes 12a, 12b have respective extended portions 13a, 13b, each with a comb shape. The comb of each of the extended portions 13a, 13b is interdigitated with the comb of the movable mass 16.
When driving signals are applied to the driving electrodes 12a, 12b so as to generate a potential difference between the driving electrodes 12a, 12b and the ground electrode 14, electrostatic force arises between the interdigitated combs of movable mass 16 and extended portions 13a, 13b, thereby driving the movable mass 16 to move. As the movable mass 16 moves, the optical shutter 17 is inserted into a gap defined by the transmitting end 20 and the receiving end 30 so as to partially intercept beams of light incident onto the optical shutter 17.
It is important for the variable optical attenuator to vary an amount of attenuation based on wavelengths of incident light beams.
Further, it is important for the variable optical attenuator to minimize variation of a power level of the attenuated light beams, such variation being caused by a disturbance such as time passing, wavelengths of the incident light beams, polarization change of the incident light beams and vibration.
However, a conventional variable optical attenuator is disadvantageous in that wavelength dependent loss (WDL) and polarization dependant loss (PDL) are great because the optical shutter has a flat panel shape.
FIGS. 2A and 2B illustrate schematic views of conventional optical shutters in accordance with the conventional art.
Referring to FIG. 2A, light beams traveling from the transmitting end 20 of an optical fiber to a receiving end 30 of an optical fiber are partially intercepted by an optical shutter 27. Here, the optical shutter 27 is formed of the same silicon material as a known actuator.
Of the light beams incident to the optical shutter 27, a great portion of light beams R is reflected by the optical shutter, so that entry of the reflected light R to the receiving end 30 is prevented. However, since the optical shutter 27 is made of silicon having high transmittance, a portion of the light beams T is allowed to be incident to the receiving end, 30 of the optical fiber through the optical shutter 27. Further, the light beams are scattered by reflection and therefore scattered lights S1, S2 are generated. Of the scattered lights, a portion S1 enters the receiving end 30 and the other portion S2 may be reflected back into the transmitting end 20. Accordingly, the conventional optical shutter 27 has a disadvantage of low light shutoff efficiency because the optical shutter 27 is made of silicon having high transmittance. Therefore, to solve a problem of low light shutoff efficiency of the optical shutter 27, an optical shutter coated with a reflective metal layer having high reflectivity (about 90%) is provided with reference to FIG. 2B. The reflective metal layer is formed of a material of Au, Ni, Cu, Al and Pt.
FIG. 2B illustrates an optical shutter 37 coated with a reflective metal layer 37a made of Au. The optical shutter 37 reflects almost of light beams R incident onto the optical shutter 37, so that few of light beams may be incident onto the receiving end 30 of the optical fiber.
However, the optical shutter 37 coated with the Au layer 37a generates scattered lights S1, S2, and the scattered lights S1, S2 enter the receiving end 30 and the transmitting end 20.
For example, in the case of attenuating light beams incident to the optical shutter coated with the Au layer to 50%, 49% of light beams R of the entire light beams passing out the transmitting end 20 may be intercepted by reflection from the Au layer on the optical shutter 37, but 1% of light beams are scattered and the scattered lights S1, S2 enter the receiving end 30 and reenter the transmitting end 20.
However, even though the scattered lights S1, S2 are few, reentry of the scattered lights S2 to the transmitting end 20 by back reflection increases. Further, the amount of the scattered lights S1, S2 is subtly changed based on wavelengths and polarization of the incident light beams. In the case that the scattered lights enter the receiving, end 30, WDL and PDL increase, thereby deteriorating attenuation reliability.
As described above, the conventional MEMS variable optical attenuator has a disadvantage of low reliability because an the amount of back reflected lights increases due to the low light shutoff efficiency of the optical shutter and because the WDL and PDL increase.